Method of image filtering based on selected pixels and a difference between pixels

ABSTRACT

A method and apparatus to remove a block effect and a ring effect appearing in a compression-coded image is disclosed. The present invention is especially applicable to an image compression-coded at a low bit rate. In particular, the present invention includes a variety of masks for the removal of the block/ring effect. Thus, one mask is select for a pixel to be filtered. Moreover, candidate pixels to be averaged with the pixel to be filtered is selected from the pixels of the selected mask to better maintain the details of the image, and a weight is applied to improve the calculation speed of the averaging operation. More particularly, the masks according to the present invention have longer tabs toward adjacent blocks at boundaries of the given block to remove the block and ring effect.In the method, a pixel of an image is filtered according to a filtering methodology. The filtering methodology includes selecting at least four pixels of successive pixels according to a position of a pixel to be filtered. The four pixels include the pixel to be filtered. At least one of the four pixels may be in a first block, and three of the four pixels may be in a second block adjacent to the first block. The filtering methodology includes determining a difference between two of the successive pixels and comparing the difference to a threshold.

DIVISIONAL REISSUE APPLICATIONS

Notice: More than one reissue application has been filed for the reissueof U.S. Pat. No. 6,594,400. The reissue applications are applicationSer. Nos. 11/102,888, 11/102,889, 11/102,890 and 11/102,891, 11/785,082,11/785,083, 11/785,084, 11/785,085, 11/785,086, 11/785,087, 11/785,088,11/785,089, 11/785,090, 11/785,091, 11/978,688, 11/978,689, 11/978,690,11/978,691, 11/978,692, 11/978,693, 11/978,694, 11/978,695, 11/978,696,11/978,697, 11/978,698, and 11/978,699, all of which are divisionalreissues of U.S. Pat. No. 6,594,400.

DOMESTIC PRIORITY INFORMATION

This is a direct divisional of application Ser. No. 11/102,890, filedApr. 11, 2005; the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a method of filtering an image decoderand more particularly, to a method of removing a block effect and ringeffect which appear in a compression-coded image of an image decoder.The present invention is especially applicable to reduce the block andring effects appearing in an image coded at a very low bit rate .

DISCUSSION OF THE RELATED ART

To encode either a still or motion image using a process based upon adiscrete cosine transform (DCT), the image is divided into a pluralityof segments. Typically, the image is divided into 8×8 segment blocks andthe DCT operation is executed for each block. However, an inter-blockcorrection information cannot be obtained by such a block-based DCToperation, thereby causing a visual discontinuity effect, known as theblock effect, to appear at boundaries between adjacent blocks.

Generally, when the DCT operation is performed on an original image,most of the significant information would be concentrated within thelower-frequency components rather than the higher-frequency components.Thus, the low-frequency components of the block-based DCT process wouldinclude a large amount of correlation information regarding adjacentblocks. However, the block-based DCT process lacks the inter-blockcorrelation information. As a result, if the low-frequency componentsare quantized on a block-by-block basis, a continuity is lost betweenadjacent blocks, resulting in the block effect in a reconstructed image.

Also, by quantizing the coefficients obtained from the DCT process, thenumber of bits is reduced. Namely, as the interval of the quantizationstep ‘q’ increases, the number of components to be coded reduces,resulting in a reduction in the number of bits. At the same time,however, the high-frequency components of the original image is arelost, causing a distortion called the ring effect in the reconstructedimage. The ring effect increases with the quantization step interval andis especially apparent in object contours of the reconstructed image.

The cause of the block effect and the ring effect may generally bedeemed as a loss of information in the original image. Moreover, with alower bit rate, the loss of information is more sever severe and, theblock effect and/or ring effect becomes more significant.

One simple method to reduce the block effect and/or the ring effect is alow pass filtering (LPF). For example, an averaging operation or adigital signal process which substantially has the effect of a LPF maybe used to remove the block effect or ring effect.

One LPF technique is based on the an averaging operation, including afiltering masking by which nine pixels are selected. Basically, a givenpixel and eight pixels surrounding the given pixel are selected as thenine (3×3) pixels. Thereafter, the nine pixels are summed and divided bynine to obtain the average pixel value. However, this LPF technique isdisadvantageous because it further filters object contours which areimportant factors for image recognition. Other LPF techniques arevariants on the application of the filtering masking in consideration ofthe form and pixels selected.

Another method to reduce the block effect and/or the ring effect is anadaptive LPF in which an image is partitioned into blocks according tothe directions of the image contours. A filter suitable to the contourdirections of the partitioned blocks is then employed. The adaptive LPFcan be applied to a local image characteristic by partitioning areconstructed image into blocks according to the object contours of thereconstructed image. However, the directions of the object contours isdifficult to find when performing a coding operation at a low bit rate.Thus, the adaptive LPF cannot be applied in a very low bit rate coding.

Other block/ring effect reduction methods include repeating processes ata frequency domain and image domain under a predetermined restriction;utilizing both the previous information of an original image and thetransmitted data to remove the block effect (POCS/CLS algorithm basedregularization); and a constrained quadratic programming. However, thesemethods cannot be applied in a real-time process because they are allrepetitive. Moreover, these methods process data at both the frequencydomain and image domain, thereby complicating the construction of acoder and decoder.

Still other methods of reducing the block/ring effect include a methodof moving the position of the 8×8 blocks to be coded by an interval ofone of two frames in successive images such that the block effectvisually appears less; a method of controlling a filtering level using afrequency analyzer; a projection method and a smoothing operationrepeating method; and a method of performing a filtering operation basedupon the quantization noise information transmitted from a coder.However, these methods all have problems. Namely, most internationalstandards prescribe that blocks have fixed positions, thus the blockposition should not be moved. Controlling a filtering level only changespixel values at boundaries between adjacent blocks, resulting in adegradation in the block effect removal. The projection method requirerequires a large amount of processing time and the filtering based uponthe quantization noise is inefficient because additional informationmust be obtained from the coder, which increases the amount of bitsbeing generated.

OBJECTIVES OF THE INVENTION

An object of the present invention is to solve at least the problems anddisadvantages of the related art.

An object of the present invention is to reduce the block and ringeffects while maintaining the details of an image. Thus, the object ofthe present invention is to enhance the picture quality of areconstructed imaged obtained by decoding a coded bit stream.

Another object of the present invention is to remove the block and ringeffects in a compression-coded image.

A further object of the present invention is to remove the block andring effects in a compression-coded imaged coded at a very low bit rate.

A still further object of the present invention is to remove theblock/ring effects in a compression-coded image which is transmitted bya block-by-block basis.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

To achieve the objects and in accordance with the purposes of theinvention, as embodied and broadly described herein, a method ofremoving the block and ring effects in a compression-coded imagetransmitted by a block-by-block basis includes comparing differencesbetween an arbitrary pixel to be filtered and the pixels surrounding thearbitrary pixel with a predetermined threshold value and selectingcandidate pixels to be associated with an averaging operation among thesurrounding pixels in accordance with the compared results; andselecting the surrounding pixels using any one of five 8-tab masksadaptive to a position of the arbitrary pixel. In four of the five 8-tabmasks, the tabs are arranged such that the upper, lower, left and rightblocks adjacent to the arbitrary block is longer, respectively. Also,one of the five 8-tab masks is a modified 3×3 filtering mask such thatone corner of the 3×3 mask is discarded.

In another embodiment of the present invention, a method of removing theblock and ring effects in a compression-coded image transmitted on ablock-by-block basis, includes adaptively selecting for a given block amask to be filtered in consideration of the directions of blocksadjacent to the given block; comparing differences between an arbitrarypixel to be filtered in the selected mask and pixels in the selectedmask surrounding the arbitrary pixel with a predetermined thresholdvalue, and selecting pixels to be associated with an averaging operationamong the surrounding pixels in accordance with the compared results;and applying a desired weight to the arbitrary pixel in consideration ofthe number of pixels excluded in the averaging operation, and performingthe averaging operation with respect to the arbitrary pixel and theselected pixels for a filtering operation with respect to the arbitrarypixel.

An image decoder according to the present invention which reconstructsan image transmitted on a block-by-block basis and filters thereconstructed image includes a filtering masking unit selecting afiltering mask including an arbitrary pixel to be filtered and pixelssurrounding the arbitrary pixel, in consideration of the position of thearbitrary pixel in a given block and in consideration of the directionsof blocks adjacent to the given block; candidate pixel selection unitcomparing differences between the arbitrary pixel and the surroundingpixels with a predetermined threshold value and selecting candidatepixels to be associated with an averaging operation among thesurrounding pixels in accordance with the compared results; andaveraging unit averaging with respect to the arbitrary pixel and thecandidate pixels selected by the candidate pixel selection unit toperform the filtering operation with respect to the arbitrary pixel.

The candidate pixel selection unit is adapted to exclude each of thesurrounding pixels from the averaging operation if a difference betweena surrounding pixel and the arbitrary pixel exceeds the predeterminedthreshold value. The averaging unit is adapted to apply a desired weightto the arbitrary pixel in consideration of the number of pixels excludedby the candidate pixel selection unit to perform the averaging operationwith respect to both the arbitrary pixel and the candidate pixelsselected by the candidate pixel selection unit.

Preferably, the threshold value is defined as δ=k×q, where δ is thethreshold value, k is a constant determined in consideration of theposition of the arbitrary pixel in the given block and the direction ofthe blocks adjacent to the given block, and q is a quantization stepinterval of the given block.

SUMMARY OF THE INVENTION

The present invention provides methodologies for filtering an image.

In an embodiment of the present invention, a pixel of an image isfiltered according to a filtering methodology. The filtering methodologyincludes selecting at least four pixels of successive pixels accordingto a position of a pixel to be filtered. The four pixels include thepixel to be filtered. At least one of the four pixels may be in a firstblock, and three of the four pixels may be in a second block adjacent tothe first block. The filtering methodology includes determining adifference between two of the successive pixels, and comparing thedifference to a threshold.

For example, in one embodiment, the difference is between two of theselected at least four pixels in the second block. Also, one of the twoselected pixels upon which the difference is based may be the pixel tobe filtered.

In another embodiment, the pixel to be filtered is in the first block.For example, the filtering methodology may includes determining thedifference between a pixel in the first block and a nearby one of thepixels in the second block.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a schematic construction of an image coder and decoder inaccordance with the present invention;

FIGS. 2(a) to 2(e) are views illustrating a variety of mask formsaccording to the present invention; and

FIG. 3 is a view illustrating mask selections based on positions of ablock.

DETAILED DESCRIPTION OF THE INVENTION

The present invention removes the block effect and ring effect appearingin a compression-coded image by an adaptive candidate pixel selection.

In the adaptive candidate pixel selection, a mask to be filtered isselected and pixels to be averaged are selected from the pixels in theselected mask. The pixels are selected by comparing differences betweena pixel to be filtered and pixels surrounding the pixel to be filteredwith a predetermined threshold value δ.

The threshold value δ is proportionate to a quantization step intervalused for the quantization of DCT coefficients in a coder. Namely, thethreshold value δ is varied according to the required objectives. Forexample, a threshold value δ used for the removal of the ring effect issmaller than the threshold value used for the removal of the blockeffect.

After the pixels to be averaged are selected, the averaging operation isperformed with respect to the selected pixels and the pixel to befiltered. At this time, a weight may be applied to the pixel to befiltered. The weight, it applied, corresponds to the number of pixelsnot selected in the selected mask.

For example, if each mask is set to include eight pixels, the weight isdetermined such that the values of pixels to be averaged can be dividedby 8 for the averaging operation even though the number of pixels issmaller than 8. Particularly, the weight determination is performed in away that the ‘division by 8’ can be executed by a simple shift command.The use of the shift command makes the averaging operation simple,resulting in a reduction in processing time and thus, a simplificationin a decoder construction.

FIG. 1 shows a schematic construction of an image coder and decoder inaccordance with the present invention. The image coder includes adiscrete cosine transform (DCT) unit 101, and a quantization unit 102.An image decoder includes a dequantization unit 103, an inverse DCT unit104, and a filtering process unit 105 to perform a filtering operationaccording the present invention.

In the coder, the original image is divided into blocks, where eachblock has, for example, an 8×8 size. The DCT unit 101 performs a DCToperation with respect to the divided blocks to generate DCTcoefficients. The quantization unit 102 quantizes the DCT coefficientsfrom the DCT unit 101 and transmits the coefficients in the form of abit stream to the decoder through a transmission channel.

The quantization operation may be performed in a variety of waysdepending upon the coding method. However, the lower-frequencycomponents generally have relatively larger amounts of significantinformation regarding the original image than higher-frequencycomponents. As a result, the lower-frequency components are quantized bya shorter quantization step interval and the higher-frequency componentsare quantized by a longer quantization step interval.

In the decoder, the DCT coefficients are dequantized by thedequantization unit 103 and inverse DCT-processed by the inverse DCTunit 104, to form a reconstructed image. Thereafter, the filteringprocess unit 105 of the present invention performs a filtering operationwith respect to the reconstructed image to obtain the resultant image,without block effect effects and ring effect effects.

Namely, the block effect effects and the ring effect effects appear inthe reconstructed image of the decoder. The ring effect is generatedwhen quantizing high-frequency components of the DCT coefficients. Thering effect is composed of spatial sinusoidal signals having an averageof ‘0’ and generated at a short period. A human's visual system issensitive to the ring effect near flat areas in which contours of animage are present. The block effect is generated by quantizinglow-frequency components of the DCT coefficients. The block effectproduces vertical and horizontal partitions in the reconstructed imageto which the human eyes are sensitive.

A filtering process of the present invention for reducing the ringeffect and the block effect includes determining a mask form, selectingsurrounding pixels to be used in the averaging operation, and performingthe averaging operation.

As discussed above, a low pass filtering is a simple method to removethe block and ring effect. In the present invention, the mask selectionis important. FIGS. 2(a) to 2(e) show a variety of mask forms to beselected in the present invention, wherein the reference character ‘r’denotes a reference pixel to be filtered.

Particularly, FIG. 2(a) shows a mask form in which eight tabs areselected from a (3×3) mask form, discarding one left upper tab. Thismask form is effective for the center of a block. Although the upperleft tab is discarded in FIG. 2(a), any one of the corner tabs may bediscarded.

FIG. 2(b) shows a mask form in which more tabs are selected in avertical direction than a horizontal direction, and more vertical lowertabs are selected than vertical upper tabs. FIG. 2(c) shows a mask formin which more tabs are selected in the vertical direction than thehorizontal direction, and more vertical upper tabs are selected thanvertical lower tabs. FIG. 2(d) shows a mask form in which more tabs areselected in the horizontal direction than the vertical direction, andmore horizontal left tabs are selected than horizontal right tabs. FIG.2(e) shows a mask form in which more tabs are selected in the horizontaldirection than the vertical direction, and more horizontal right tabsare selected than horizontal left tabs.

In FIGS. 2(a) to 2(e), eight tabs are utilized in each of the five masksin consideration of high-speed operation and constructionsimplification, as will be discussed below. Also, in the preferredembodiment, the five mask forms shown in FIGS. 2(a) to 2(e) are assignedto positions corresponding to the letter of the Figure in an 8×8 block,as shown in FIG. 3.

The mask form in FIG. 2(c) is used in the upper boundary of the blockbecause of the long vertical tabs. This allows an averaging of the pixelto be filtered with a maximum of three pixels in the upper adjacentblock. The mask form shown in FIG. 2(b) is used in the lower boundary sothat the pixel to be filtered is averaged with maximum of three pixelsin the lower adjacent block. The mask form shown in FIG. 2(d) is used inthe left boundary so that the pixel to be filtered is averaged withmaximum of three pixels in the left adjacent block. The mask form shownin FIG. 2(e) is used in the right boundary so that the pixel to befiltered is averaged with maximum of three pixels in the right adjacentblock. Finally, the mask form shown in FIG. 2(a) is used at diagonallyfrom the corners to the center of the 8×8 block so what the pixel to thefiltered is averaged with the pixels from the adjacent blocks.

As discussed above, utilizing eight tabs in each of the five masks makesthe averaging operation and decoder construction simple, whilemaintaining the filtering effect. Namely, an averaging operation basedon a typical 3×3 mask includes eight additions and one division, whichrequires a large amount of calculation time. However, a modified 3×3mask according to the present invention maintains a denominator at ‘8’and a ‘division by 8’ can be executed by a simple shift command whichrequires a small amount of calculation time. As a result, the divisioncan be substituted with a shift operator.

After selecting an appropriate mask, candidate pixels to be averaged maybe selected. Rather than averaging all the pixels for a pixel to befiltered, some pixels are excluded from the averaging operation. Thepixels are selected based upon a comparison between the difference ofthe pixel to be filtered from a pixel corresponding to an object contourand the predetermined threshold value δ. Namely, an absolute value ofsuch difference is compared with the predetermined threshold value δ.

If a given pixel is averaged together with pixels forming the objectcontours, the details of an image are filtered to vanish. Thus, aprocess of excluding pixels determine as object contours from theaveraging operation for a pixel to be filtered would reduce the blockand ring effects while maintaining the image details.

Accordingly, a pixel of the selected mask is included in the averagingoperation if the difference, i.e. the absolute value, between the pixelof the selected mask and a pixel to be filtered is smaller than thepredetermined threshold value δ. If the difference is not smaller, thepixel of the selected mask is excluded from the averaging operation.

Also, to set the denominator of the averaging operation to 8, anappropriate weight is applied to the pixel to be filtered, inconsideration of the number of pixels excluded in the averagingoperation. For example, if two surrounding pixels are excluded in theaveraging operation for a pixel P_(i), a weight compensating the twopixels are added to the original value of P_(i). Specifically, a weightof 2×P_(i) would be added to the P_(i), making the value of the pixel tobe filtered 3×P_(i). Thus, the weight added to the pixel to be filteredis the value of the pixel to be filtered multiplied by the number ofpixels excluded.

The threshold value δ is determined in consideration of the block effectand the ring effect. Because the degrees of the block/ring effects arare proportionate to the quantization step interval, the threshold valueδ is defined as follows:δ=k×qwhere, k is a constant and q is the quantization step interval of ablock.

In the preferred an example embodiment of the present invention, thevalue k is 1.0 for filtereing filtering pixels at the boundary of the ablock and 0.6 for filtering pixels within the boundary pixels of theblock. The value of k is greater at the block boundaries to cope moreefficiently with the block effect.

Thus, the preferred example embodiment of the present filteringoperation includes selecting an appropriate mask, selecting pixels to beaveraged with a pixel to be filtered from the selected mask, andaveraging the selected pixels and the pixel to be filtered. Any one ofthe five 8-tab masks shown in FIGS. 2(a) to 2(e) are selected dependingupon the position of the pixel to be filtered in a block, and the pixelsin the selected mask, excluding pixels determined as object contours,are averaged with the pixel to be filtered. Moreover, by applying aweight to the pixel to be filtered, the denominator of the averagingoperation may be kept constant. Accordingly, the present filteringoperation reduces the block effect and the ring effect.

As discussed above, according to the present invention, a determinationis made whether pixels surrounding the pixel to be filtered is are partof the object contour and an averaging operation is performed based uponthe determined result. A modified 3×3 filtering mask is used within agiven block to remove the ring effect. Particularly, the masks includesmore tabs toward adjacent blocks at boundaries of the given block toremove the block effect and ring effect.

Moreover, pixels to be included in the averaging operation for thefiltering operation and pixels to be excluded therefrom are alsodetermined based upon a threshold value, and a weight is applied to apixel being filtered in the averaging operation in consideration of thenumber of the excluded pixels such that the denominator remains ‘8’ inthe averaging operation. Therefore, the present invention not onlyreduces the block and ring effects, but reduces the processing time witha simple hardware construction.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teachings canbe readily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims invention. Many alternatives, modifications, andvariations will be apparent to those skilled in the art.

1. A filtering method for a pixel P of a block B in a reconstructed image, comprising: selecting one of a plurality of filtering masks based upon a position of said pixel P in said block B; and averaging said pixel P and candidate pixels within the selected filtering mask.
 2. A method of claim 1, wherein each of the plurality of filtering masks has 8 tabs.
 3. A method of claim 1, wherein a filtering mask with more tabs toward a block adjacent said block B is selected.
 4. A method of claim 1, further comprising selecting a pixel within the selected filtering mask P_(m) as a candidate pixel if the value of the pixel P_(m) meets a predetermined condition.
 5. A method of claim 4, wherein the pixel P_(m) is selected as a candidate pixel if the absolute value of the difference between said pixel P and pixel P_(m) is less than a threshold value.
 6. A method of claim 5, wherein the threshold value is calculated by an equation below, δ=k×q where k is a constant and q is a quantization step interval of said block B.
 7. A method of claim 6, wherein the value of k is 1.0 for filtering boundary pixels of said block B and 0.6 for filtering pixels within the boundary pixels of said block B.
 8. A method of claim 4, further comprising adding a weight value to said pixel P prior to the averaging, wherein said weight value is based upon a number of pixels P_(m) not selected as candidate pixels.
 9. A method of claim 8, wherein said weight value is the number of pixels P_(m) not selected multiplied by the value of said pixel P.
 10. A filtering apparatus to filter a pixel P of a block B in a reconstructed image, comprising: a filtering masking unit selecting one of a plurality filtering masks based upon a position of said pixel P in said block B; and an averaging unit averaging said pixel P and candidate pixels within the selected mask.
 11. An apparatus of claim 10, wherein each of the plurality of filtering masks has 8 tabs.
 12. An apparatus of claim 11, wherein the plurality of filtereing masks are modified 3×3 mask forms including: a filtering mask in which eight tabs are selected from the 3×3 mask form, discarding one corner tab; a filtering mask in which more tabs are selected in a vertical direction than a horizontal direction, and more vertical lower tabs are selected than vertical upper tabs; a filtering mask in which more tabs are selected in the vertical direction than the horizontal direction, and more vertical upper tabs are selected than vertical lower tabs; a filtering mask in which more tabs are selected in the horizontal direction than the vertical direction, and more horizontal left tabs are selected than horizontal right tabs; and a filtering mask in which more tabs are selected in the horizontal direction than the vertical direction, and more horizontal right tabs are selected than horizontal left tabs.
 13. An apparatus of claim 10, wherein a filtering mask with more tabs toward a block adjacent said block B is selected.
 14. An apparatus of claim 10, further comprising: a comparison unit selecting a pixel within the selected mask P_(m) as a candidate pixel if the value of the pixel P_(m) meets a predetermined condition.
 15. An apparatus of claim 14, wherein the pixel P_(m) is selected as a candidate pixel if the absolute value of the difference between said pixel P and pixel P_(m) is less than a threshold value.
 16. An apparatus of claim 15, wherein the threshold value is calculated by an equation below, δ=k×q where k is a constant and q is a quantization step interval of said block B.
 17. An apparatus of claim 16, wherein the value of k is 1.0 for filtering boundary pixels of said block B and 0.6 for filtering pixels within the boundary pixels of said block.
 18. An apparatus of claim 14, wherein the averaging unit adds a weight value to said pixel P prior to the averaging, wherein said weight value is based upon a number of pixels P_(m) not selected as candidate pixels.
 19. An apparatus of claim 18, wherein said weight value is the number of pixels P_(m) not selected multiplied by the value of said pixel P.
 20. A coding and decoding method comprising: a discrete cosine transform (DCT) unit performing a DCT operation with respect to divided blocks of an image to generate DCT coefficients; a quantization unit quantizing the DCT coefficients and transmitting the DCT coefficients in a form of a bit stream through a transmission channel; a dequantization unit dequantizing the DCT coefficients received through the transmission channel; an inverse DCT unit performing an inverse DCT operation with respect to the dequantized DCT coefficients to form a reconstructed image, and a filtering process unit filtering each pixel of each block of the reconstructed image by selecting one of a plurality of filtering masks based upon a position of said pixel in said block; and averaging said pixel and candidate pixels within the selected filtering mask.
 21. A method of filtering an image, comprising: filtering, with a filtering unit, a pixel of an image according to a filtering methodology, the filtering methodology including, selecting at least four pixels of successive pixels according to a position of a pixel to be filtered, the selected at least four pixels including the pixel to be filtered, at least one of the selected at least four pixels being in a first block, and three of the selected at least four pixels being in a second block adjacent to the first block, determining a difference between two of the selected at least four successive pixels, the two of the selected at least four pixels including non-adjacent pixels, and comparing the difference to a threshold.
 22. The method of claim 21, wherein the difference is between two of the selected at least four pixels in the second block.
 23. The method of claim 22, wherein one of the two selected pixels upon which the difference is based is the pixel to be filtered.
 24. The method of claim 22, wherein the difference is based on the pixel to be filtered and a nearby pixel.
 25. The method of claim 21, wherein one of the two selected pixels upon which the difference is based is the pixel to be filtered.
 26. The method of claim 25, wherein the pixel to be filtered is in the first block.
 27. The method of claim 26, wherein one of the two successive pixels upon which the difference is based is in the second block.
 28. The method of claim 21, wherein the filtering methodology includes determining the difference between a pixel in the first block and a nearby one of the pixels in the second block.
 29. The method of claim 28, wherein the pixel in the first block is the pixel to be filtered.
 30. The method of claim 28, wherein one of the pixels in the second block is the pixel to be filtered. 